1. Field of the Invention
The present invention relates to a method for wavelength stabilization of a laser, particularly a semiconductor laser.
2. Description of the Related Art
Laser modules containing semiconductor lasers for optical transmission systems with wavelength division multiplex technology (WDM technology) must hold their wavelength very stable during their entire required lifetime (105 hours!), so that the transmitting characteristics in the wavelength channel do not change unacceptably, or respectively, so that crosstalk onto neighboring channels does not occur. At the moment, WDM systems with 4 and 8 wavelength channels with a channel interval of 400 Ghz (which corresponds to 3.2 nm), or respectively, 200 Ghz (which corresponds to 1.6 nm) are used. However, the number of wavelength channels will shortly increase to 16, and to 32 to 64 channels in the medium term, the channel spacing growing smaller according to the rising number of channels.
In the laser modules used today, the fine adjustment and stabilization of the wavelength is effected exclusively via the temperature of the laser diode. The typical change in the wavelength for 1.5-xcexcm semiconductor lasers is 0.1 nm/K of the temperature change, for example. This indirect wavelength stabilization has the disadvantage that it does not take aging effects of the laser diode into account. Leading manufacturers of laser modules currently guarantee a wavelength stability of 0.3 nm within the lifetime. This value is not sufficient for future high-performance WDM transmission systems with a smaller channel spacing, however.
By contrast, the invention proposed here has the advantage that a simple, direct wavelength stabilization of a semiconductor laser is made available on the basis of ratio control, it being possible to use this stabilization particularly in laser modules containing semiconductor lasers for optical transmission systems with WDM technology, though it is not limited to such lasers and can be used with essentially any type of laser.
In order to reliably measure the emitted wavelength, an optical filter is additionally inventively used, which can be advantageously installed in a laser module. Since the filter is a passive component, a high long-term wavelength stability can generally be achieved.
Preferable and advantageous developments of the inventive method provide for wavelength stabilization of a laser, particularly a semiconductor laser, in that a power portion of a total optical power is fed to an optical filter which is wavelength-selective and which is set at a wavelength that is to be stabilized, which filter filters out a power containing essentially only this wavelength from this supplied optical power portion, an additional power portion is derived from the total optical power emitted by the laser such that a ratio between the filtered-out power and this additional power portion is independent of this total power, the filtered-out and additional power portion are respectively measured, the ratio is formed between the measured filtered-out power portion and the measured additional power portion, the formed ratio is compared as an actual value to an adjustable desired value of this ratio, and given the deviation of the actual value from the respectively set desired value, an operating parameter of the laser on which on the wavelength to be stabilized depends is so adjusted that the actual value essentially matches the adjusted desired value.
An improvement provides that a power portion to be filtered, which is proportional to the total power, is fed to the filter, and a part of the supplied power portion to be filtered which is emitted by the filter in addition to the filtered-out power is used as an additional power portion, whereby the sum of this additional power portion and the filtered-out power is equal to or at least proportional to the supplied power portion that is to be filtered. It is preferred that a filter selected from the group of optical interference filters and Bragg gratings is used as the filter. Alternatively, a filter selected from the group of wavelength-selective optical directional couplers and interferometers is used as the filter to an output port.
In one embodiment, a power portion to be filtered out, which is proportional to the total optical power of the laser, is fed to the filter; the power which is filtered out by this filter and which essentially contains only the wavelength to be stabilized is fed to one detector; and a power portion which is derived from the total power and which is proportional to this total power is fed to the additional detector as an additional power portion, this power portion being separated from the power portion to be filtered by the filter and remaining unfiltered. According to a development of the invention, the temperature of the laser is adjusted as the operating parameter.
Preferable and advantageous developments of the inventive arrangement provide for a wavelength-selective optical filter which is set at the wavelength to be stabilized, to which filter a power portion, which is to be filtered out, of the total optical power emitted by the laser is fed and which filters out of this supplied power portion a power essentially containing only the wavelength to be stabilized, an optical detector, to which the filtered-out power is fed for purposes of detection, an additional optical detector, to which an additional optical power portion is fed for detection, this power portion being so derived from the emitted total optical power that a ratio between the filtered-out power and the additional power portion is independent of the total power, a means for forming the ratio between the detected filtered-out power and the detected additional power portion, and a means for comparing the formed ratio as an actual value to an adjustable desired value of this ratio and for adjusting an operating parameter of the laser on which the wavelength to be stabilized depends, given a deviation of the actual value from the adjusted desired value, such that the actual value essentially agrees with the adjusted desired value.
In a preferred embodiment, a power portion that is to be filtered which is proportional to the total power is fed to the filter, and a part of the supplied power portion to be filtered which is emitted by the filter in addition to the filtered-out power is conducted to the additional detector as an additional power portion, whereby the sum of this additional power portion and the filtered-out power is equal to, or at least proportional to, the supplied power portion to be filtered.
The filter consists of a filter selected from the group of optical interference filters and Bragg gratings, the power which is filtered out from the supplied power portion to be filtered and which essentially contains only the wavelength to be stabilized is fed to a detector, and the remaining part of the supplied power portion to be filtered, which part is emitted by this filter and is essentially free of the wavelength to be stabilized, is fed to the additional detector as the additional power portion. Alternatively, the filter consists of a filter selected from the group of wavelength-selective optical directional couplers and interferometers, whereby this filter comprises an input port for purposes of coupling into the filter the supplied power portion to be filtered; an output port for coupling out of the filter a power that is filtered out of the power portion that was coupled in for filtering and that essentially contains only the wavelength to be stabilized; and an additional output port for purposes of coupling out of the filter the remaining part of the power portion that was coupled in for filtering, which part forms the additional power portion, and the power that is filtered out from the output port is fed to the detector, and the additional power portion, which is coupled out from the other output port, is fed to the additional detector.
Preferably, a power portion to be filtered, which is proportional to the total optical power of the laser, is fed to the filter; the power, which is filtered out by this filter from supplied power portion that is to be filtered and which essentially contains only the wavelength to be stabilized, is fed to one detector; and a power portion which is derived from the total power and which is proportional to this total power is fed to the additional detector as an additional power portion, this power portion being separated from the power portion to be filtered by the filter and remaining unfiltered by this filter. A wavelength-neutral optical power divider is provided, to which an optical power of the laser that is proportional to the total power is fed and which generates two power portions from this supplied power, one of which is fed to the filter as the power portion to be filtered.
In one embodiment, the laser comprises two light exit windows for purposes of respectively emitting a respective subpower of the total optical power generated by the laser, whereby the subpower from a light exit window is intended for use; the subpower from the other light exit window, which is proportional to the total power, is fed to the filter as the power portion to be filtered; the filter splits the supplied power portion that is to be filtered into the filtered-out power, which essentially contains only the wavelength to be stabilized, and the remaining part of this supplied power portion that is to be filtered, which part forms the additional power portion; and the filtered-out power is fed to the detector and the additional power portion is separated from the filtered-out power and fed to the additional detector.
In one embodiment, the laser comprises two light exit windows for purposes of respectively emitting a respective subpower of the total optical power generated by the laser, whereby the subpower from a light exit window is intended for use; the subpower from the other light exit window, which is proportional to the total power, is fed to the wavelength-neutral power divider, which generates two power portions from this subpower, one of which is fed to the filter as the power portion to be filtered out, and the other of which is fed to the additional detector as the additional power portion; and the power portion that is filtered out from the supplied power portion which is to be filtered, which essentially contains only the wavelength that is to be stabilized, is fed to a detector. Alternately, the laser comprises two light exit windows for purposes of respectively emitting a respective subpower of the total optical power generated by the laser, whereby the subpower from a light exit window is intended for use; the subpower from the light exit window which is intended for use is fed to the wavelength-neutral power divider, which generates two power portions from this subpower, one of which is fed to the filter as the power portion to be filtered out, and the other of which is available for use, whereby the power which is filtered by the filter out of the supplied power portion that is to be filtered and which essentially contains only the wavelength to be stabilized is fed to the detector; and the subpower from the other light exit window, which is proportional to the total power, is fed to the additional detector as the additional power portion.
The laser of another embodiment comprises a light exit window for purposes of emitting an optical power that is proportional to the total power, said optical power being intended for use, this generated power portion is fed to an additional wavelength-neutral power divider, which, in turn, splits this power portion into two power portions, one of which is intended for use; the other power portion that is generated by one of the two power dividers is fed to the filter for filtering, and the power, which is filtered out by this filter from this power portion that is to be filtered and which essentially contains only the wavelength to be stabilized, is fed to the detector; and the additional power portion generated by the other power divider is fed to the additional detector as the additional power portion.
In the present apparatus, the optical power may either penetrate an optical lens or is deflected by a deflecting mirror. A particular advantage of the inventive arrangement can be seen in the design on which it is based, which permits an uncomplicated integration with a semiconductor laser, or in a whole transmission module on the surface of a substrate, using simple conventional production techniques.
The invention is preferably used in optical transmission modules with semiconductor lasers for purposes of long-term stabilization of a wavelength of the laser. Optical fixed-frequency sources for sensor technology can advantageously be realized with the invention.